Magnetic disk drives are information storage devices that use thin film magnetic media to store data. A typical disk drive as illustrated in FIG. 7 includes one or more rotatable disks 16 having concentric data tracks wherein data is read or written. Sectors are written into the disk to dissect the tracks in a spoke pattern. As a disk rotates, a transducer, also referred to as a magnetic recording head, is supported by a slider and positioned by an actuator to magnetically read data from or write data to various tracks on the disk. Typically, a transducer is attached to a slider having an air-bearing surface which is supported adjacent to a data surface comprising the data tracks by a cushion of air generated by the rotating disk. Wires typically connect the transducer on the slider to a data processing unit that controls read/write electronic circuitry.
It is known to have an actuator arm 24 comprising a slider 5 to be attached on a side opposite the air-bearing surface of a load beam 3. A pivot motor 20, which is controlled by a servo control system, rotates the actuator arm to position the magnetic head over a desired data track on the magnetic disk. A load or force is typically applied against the slider by the load beam thereby biasing the heads toward the magnetic disk so as to move the heads closer to the magnetic disk. This force is compensated for by a cushion of air between the slider air bearing surface and the rotating disk.
In current disk drives servo information is written on sectors on each disk. Typically, a disk is divided into 80 sectors. A servo burst containing read-only servo encoded position information is embedded where a sector intersects a track. While reading or writing data on a track the head passes over the servo bursts and depending upon the location of the read/write head in relation to the servo burst a signal may be generated to bring the head into alignment with the track. The burst signal actuates the pivot motor. If the read/write head is properly aligned with the servo burst, no signal is generated. This type of servo is referred to as an embedded servo or sector servo.
During operation a relative position of the head may be displaced minutely due to heat generated in the disk drive or a change in ambient temperature. This minute displacement can cause the magnetic recording heads to be positioned off the signal tracks designated by the servo-positioning head. Mispositioning can cause delay, carried to an extreme mispositioning can cause errors in reading the data or writing signals to the magnetic disk drive. In order to correct a displacement of the read/write head, power must be applied to the servo actuator. An amount of displacement to be corrected is on the magnitude of micrometers. It is difficult to move a head by this small amount accurately by controlling the power of the servo actuator. Therefore, the conventional magnetic heads supporting mechanism is not suitable to meet recent requirements of high-density recording. In addition, in the case where the writing or reading operation is carried out while switching a plurality of data heads sequentially in a data region allotted by a cylinder, the position of the data head needs to be corrected each time the data head is switched. Switching prevents a continuous writing or reading operation from being carried out. This necessitates a waiting time of about 10 to 15 microseconds in order to turn the recording disk until a target sector aligns again thereby lowering a throughput. In order to address these problems of individual data head alignment, microactuators have been useful in aligning each data head over an appropriate data track.
Microactuator designs are known to utilize piezo elements mounted on panels to provide cross track motion to a read/write gap by bending in a plane FIG. 1. Typically, two piezo elements are mounted in parallel and perpendicular to the plane of a load beam. As the piezo crystals are activated, they contract or elongate in unison bending the elements. FIG. 4. The elongation or contraction causes a beam like element to move as the elements bend. Such configurations are particularly attractive since the bending energy provides sufficient cross track motion with a piezo driven microelectronic machine (MEM) juxtaposed to the slider. The panels are designed to flex in a direction that coincides with the motion of a read/write element as it seeks from track to track. In addition, these panels provide structural stiffness.
The bending of the piezo crystals provides cross track motion to a slider that is attached via rigid links to these panels at the antinodes. Thus, during bending the slider moves by an amount that is equal to an amount of bending reflection in the piezo crystals. A piezo crystal so mounted has the benefit of providing additional pitch and torsional stiffness. Also, the piezo crystals can be mounted on a thin sheet of stainless steel to provide reinforcement to the piezo crystals. Juxtaposition of the MEM and slider is highly desirable to dynamically de-couple the microactuator from the load beam. These concepts utilizing bending are efficient and reliable, however, the physical size of the design limits the use of known MEM microactuators with vertical mounted piezo crystals to disk drives with sufficient space to house a larger disk drive. This constraint limits their usefulness to desktop sized products or larger systems. A need still exists for microactuator designed suitable for use in smaller disk drives such as those used with smaller spacing between the disks.
In addition a microactuator located away from the slider has associated with it some performance penalties. When a microactuator MEM is mounted very close to the mounting plate of the suspension and in some cases becomes an integral part of the mounting plate away from the slider, dynamic signatures of the microactuator and the main actuator are not easily distinguishable from one another. Therefore, it is difficult to control these two mechanisms independently.
A further drawback of the earlier designs includes torsional modes of a load beam that can give rise to off track motion. In addition, gain associated with the first sway mode can be unexpectedly high, in the range of 22 dB for a sway frequency of 5 Khz and to 35 dB for a sway frequency of 9 Khz. Such high gains make clipping of such frequencies necessary.
Present needs for a disk drive require a microactuator design wherein the decay rate of the asymptote must be less than 20 dB per decade with a safety margin of 5 dB required. Further, a cross over frequency must not be less than 1.6 Khz. A high gain may be clipped or notched out if the location of resonant peaks does not vary significantly, however, often this is the not situation. Natural frequencies can shift from assembly to assembly making the task of notching or clipping almost impossible.
Consequently, there is a need to provide a microactuator capable of meeting these electrical requirements and also the physical size requirements for today's smaller hard drives.